Field of the Invention
The present invention relates to a fuel cell stack including a plurality of power generation units. The power generation units are formed by an even number of electrolyte electrode assemblies and metal separators stacked alternately with the electrolyte electrode assemblies. Each of the electrolyte electrode assemblies includes a cathode, an anode, and an electrolyte interposed between the cathode and the anode. The power generation units have an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode and a fuel gas flow field for supplying a fuel gas to the anode. A buffer having an uneven shape is formed at least at one of a flow field outlet and a flow field inlet of the oxygen-containing gas flow field and at least at one of a flow field outlet and a flow field inlet of the fuel gas flow field. A coolant flow field is formed in each space between the power generation units.
Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators to form a unit cell. In use, normally a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field is formed in a surface of one separator facing the anode for supplying a fuel gas to the anode, and an oxygen-containing gas flow field is formed in a surface of the other separator facing the cathode for supplying an oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along surfaces of the separators.
In the case where metal separators are used as the separators, by providing grooves as a fuel gas flow field on one surface of the metal separator facing the anode, ridges as the back side of the grooves are formed on the other surface of the metal separator. Further, by forming grooves as an oxygen-containing gas flow field on one surface of the metal separator facing the cathode, ridges as the back side of the grooves are formed on the other surface of the metal separator.
For example, as disclosed in Japanese Laid-Open Patent Publication No. 08-222237, in a fuel cell stack formed by stacking a plurality of fuel cells each including a solid electrolyte and electrodes on both sides of the solid electrolyte, a known fuel cell separator interposed between the fuel cells has fuel gas flow grooves on one surface for supplying a fuel gas to one of the adjacent fuel cells, and has an oxygen-containing gas flow grooves on the other surface for supplying an oxygen-containing gas to the other of the adjacent fuel cells.
This separator is made of metal material having good workability, and material having good electrical conductivity is coated on both of front and back sides of the metal material. Further, a large number of projections are provided on front and back surfaces of the separator at suitable intervals. In the fuel cell stack, the projections contact the fuel cell surface, and the fuel gas flow grooves and the oxygen-containing gas flow grooves are formed between the separators and the adjacent fuel cells, respectively, and between the projections.
The fuel cell stack may adopt so called skip cooling structure where the coolant flow field is formed at intervals of a predetermined number of unit cells. In the case where the above conventional technique is adopted in the fuel cell having the skip cooling structure of this type, as shown in FIG. 6, cell units 3 each including two MEAs 1a, 1b, and three metal separators 2a, 2b, 2c are stacked together.
Each of the MEA 1a, 1b includes an anode 4b, a cathode 4c, and a solid electrolyte membrane 4a interposed between the anode 4b and the cathode 4c. The metal separator 2a has a plurality of ridges 5a forming a fuel gas flow field 5 for supplying a fuel gas to the anode 4b of the MEA 1a. The metal separator 2b has a plurality of ridges 6a forming an oxygen-containing gas flow field 6 for supplying an oxygen-containing gas to the cathode 4c of the MEA 1a and a plurality of ridges 5a forming a fuel gas flow field 5 for supplying a fuel gas to the anode 4b of the MEA 1b alternately.
The metal separator 2c has a plurality of ridges 6a forming an oxygen-containing gas flow field 6 for supplying an oxygen-containing gas to the cathode 4c of the MEA 1b. A coolant flow field 7 for supplying a coolant is formed between the adjacent metal separators 2a, 2c. 
In the metal separators 2a, 2b, the MEA 1a is sandwiched between the ridges 5a, 6a provided at the same positions, i.e., in alignment with each other in the stacking direction. In the metal separator 2b, 2c, the MEA 1b is sandwiched between the ridges 5a, 6a provided at the same position, i.e., in alignment with each other in the stacking direction.
However, though the coolant flow field 7 is formed between the cell units 3, since the ridges and grooves are provided oppositely in the stacking direction in the coolant flow field 7, the metal separators 2c, 2a are not fixedly positioned in the stacking direction. In the structure, the load at the time of stacking components of the fuel cell stack cannot be supported between the cell units 3. Further, the coolant flow field 7 is not tolerant of pressure change during power generation.
Further, the fuel cell stack may be damaged undesirably due to deformation of the MEA 1a, 1b and the metal separators 2a to 2c. Accordingly, electrical conductance between the cell units 3 is poor.